A method of forming a layer of a predetermined material by evaporating a raw material of the predetermined material is widely used in the manufacture of semiconductor means, flat panel display means, and other electronic means. A description will be given hereinbelow using an organic EL display device as one example of those electronic means. The organic EL display device having a sufficient brightness and a lifetime of several tens of thousands of hours or more uses an organic EL element being a self-light-emitting element and, thus, since peripheral components such as a backlight are small in number, it can be formed thin, and therefore, it is ideal as a flat panel display device.
The organic EL element constituting such an organic EL display device is required in terms of characteristics as a display device such that, while being a large screen, the element lifetime is long, there is no variation in luminous brightness in the screen and element lifetime, and there is no defect such as, typically, a dark spot. In order to satisfy such requirements, the organic EL film forming technique is quite important.
For example, as a film forming apparatus for uniformly forming an organic EL film on a large substrate of about 20 inches, use is made of an apparatus described in Patent Document 1 (Japanese Unexamined Patent Application Publication (JP-A) No. 2004-79904) or the like. The film forming apparatus of Patent Document 1 aims to achieve uniformity in film thickness on a large substrate by optimally arranging, in a tree fashion, a piping structure inside an injector disposed in the apparatus so as to uniformly disperse a raw material gas on the substrate along with a carrier gas.
Recently, an increase in size of 20 inches or more has also been required for this type of organic EL device. However, in order to respond to such a requirement, it is necessary to overcome various drawbacks peculiar to the organic EL device that is poor in light emitting efficiency and short in lifetime. Herein, since various organic EL films, including a light emitting layer, forming the organic EL device are as extremely thin as several tens of nm as compared with films formed in other display devices, a technique of forming a film on a molecular basis is required and, further, it is also quite important to perform the film formation on the molecular basis with high accuracy.
As a film forming apparatus also applicable to the increase in size of 20 inches or more, the present inventors have proposed, in Japanese Patent Application No. 2005-110760 (Prior Application 1), a film forming apparatus for uniformly and quickly forming a film of each of various organic EL raw materials forming an organic EL device.
The proposed film forming apparatus comprises two raw material containers for vaporizing/evaporating the same organic EL raw material, an ejection vessel for ejecting the organic EL raw material onto a substrate, and a piping system (i.e. flow paths) connecting the raw material containers and the ejection vessel to each other. In this case, when supplying the organic EL raw material to the ejection vessel from one of the raw material containers, the piping system including valves and orifices is switched in mode before the start of the film formation, at the time of the film formation, and at the time of stopping the film formation and the temperature of the piping system is controlled. In this structure, during the time other than the film formation, a gas remaining in the piping system is quickly exhausted and a gas is circulated to the other raw material container.
In the film forming apparatus shown in Prior Application 1, it is possible to prevent contamination due to the gas remaining in the piping system and further to quickly perform the state transition before the start of the film formation, at the time of the film formation, and at the time of stopping the film formation. Since the contamination due to the organic EL material remaining in the piping system can be prevented, the film forming apparatus according to Prior Application 1 can significantly improve the brightness and lifetime of an organic EL device.
However, it has been found out that when the structure shown in Prior Application 1 is employed, it is necessary to further improve the use efficiency of the organic EL material forming a light emitting layer or the like of an organic EL device and, for a further increase in size of an organic EL device, it is necessary to further improve the brightness of an organic EL element and to achieve an increase in lifetime of the organic EL element.
Further, in the film forming apparatus shown in Prior Application 1, the evaporated organic EL material is blown into the ejection vessel from one of the raw material containers during the film formation, but is exhausted to the exterior from the one of the raw material containers during the time other than the film formation. In this manner, the organic EL material is effectively used only during the film formation but is not effectively used during the time other than the film formation and, therefore, there has also been found out a drawback that the use efficiency of the using organic EL material is low.
An explanation will be given here of the characteristics and structure of an organic EL device aimed at by this invention. At first, the organic EL device aimed at by this invention is an organic EL device having a long lifetime of 10000 hours or more and a light emitting efficiency of 100 Im/V or more. To briefly explain the structure of the organic EL device according to this invention, it comprises, on a glass substrate, an anode in the form of a transparent conductive film and a cathode made of Li/Ag or the like and provided so as to face the anode, and a plurality of layers, for example, seven or five organic layers, disposed between the anode and the cathode. Herein, the organic layers are, for example, in the form of an electron injection layer, an electron transport layer, a light emitting layer, a hole transport layer, and a hole injection layer from the cathode side. The light emitting layer comprises, for example, a red light emitting layer, a green light emitting layer, and a blue light emitting layer and, by forming the red light emitting layer, the green light emitting layer, and the blue light emitting layer into a laminated structure in this manner, it is possible to emit white light with high efficiency.
Among the above organic layers, particularly the red light emitting layer, the green light emitting layer, and the blue light emitting layer forming the light emitting layer each have a thickness of about 20 nm and even the electron transport layer and the hole transport layer each have a thickness of about 50 nm. In this manner, the organic layers of the organic EL device are extremely thin as compared with the thicknesses of various films of other semiconductor devices, but, for future, an attempt is made to further reduce the thicknesses of these organic layers. In order to deposit/form an extremely thin organic layer with high accuracy, there is required an ultraprecise technology for forming a raw material of an organic layer on a molecular basis. Consequently, this means that contamination even on a molecular basis is not allowed for formation of an organic layer.    Patent Document 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2004-79904